There are various situations where there is a need for the monitoring over a period of time of the state of a live population (whether human, animal or plant) and the location of the population members. An example is where emergency service personnel are dealing with hazardous incidents. The monitoring is on the whole done by the personnel using radio or telephone communications to provide the necessary information to another human. The information provided can be analysed manually or from input to a computer, so that appropriate action can be triggered, for example in a medical emergency to provide appropriate help to the correct location. This system relies on a human initiating the communication in the first place, and may not provide accurate medical or location information. It is also difficult to correlate information from different sources to provide an overall picture of the incident, to enable hazards to be predicted.